Mixed alloy lead-free solder paste

ABSTRACT

This invention is to a mixed alloy lead-free solder paste, a method of making the solder paste, and a method of using the solder paste. The solder paste comprises particles of a first alloy and a second alloy, mixed in a flux. The liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than about 15° C.

RELATED APPLICATION

This application is a Divisional of Ser. No. 10/334,132, filed on Dec. 31, 2002.

FIELD OF THE INVENTION

This invention is directed to a mixed alloy lead-free solder paste, as well as a method of making and using the solder paste. More specifically, the mixed alloy is a combination of at least two types of alloy particles having liquidus temperatures that reduce or eliminate tombstoning effects during the soldering process.

BACKGROUND OF THE INVENTION

As the electronics industry is forced to eliminate lead from consumer products, there is an increasing challenge to create new environmentally friendly solder paste. The new lead-free solder comes with new challenges.

U.S. Patent Publication No. U.S. 2002/0040624 A1 discloses a solder paste which is comprised of a lead-free solder alloy powder and a flux. The solder alloy powder is a mixture of from 10 to 30 vol % of a first powder of a Sn—Bi alloy consisting essentially of 10-45 wt % of Bi and a balance of Sn and from 70 to 90 vol % of a second powder of a Sn—Zn alloy consisting essentially of 9-15 wt % of Zn and a balance of Sn. The mixture gives an alloy having a composition upon melting which consists essentially of 7-11 wt % of Zn, 1-5 wt % of Bi, and a balance of Sn.

U.S. Pat. No. 6,360,939 B1 discloses a method of manufacturing a lead-free electrical solder paste having a primary solder powder and an additive metal powder component that does not melt during the soldering process. The primary powder is the same as is used in conventional solder paste. The additive powder has a melting point substantially higher than the melting point of the primary powder. The primary powder comprises between 80-99% Sn and 1-20% Ag. The additive powder metal is selected from the metal group comprising Sn, Ni, Cu, Ag, and Bi and mixtures thereof.

One problem associated with soldering is tombstoning. Tombstoning is generally defined as the displacement of an electrical component during reflow soldering into a somewhat vertical position. In other words, tombstoning is where one end of an electrical component is displaced in an upward position as that component is soldered to a substrate. The problem is of particular concern in lead-free soldering as lead-free materials have significantly higher coalescent force (wetting force) which promotes tombstoning. It would, therefore, be of great advantage to find a lead-free solder material that substantially reduces or eliminates tomb stoning.

SUMMARY OF THE INVENTION

The solder paste of this invention is one that provides a homogeneous solder material, and which substantially reduces or eliminates undesirable tombstoning effects during the reflow process. In one embodiment, the invention provides a mixed alloy solder paste, which comprises particles of a first alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C., and particles of a second alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C. Also included in the solder paste is flux. Preferably, the particles of the first and second alloy are mixed with the flux and the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 15° C.

In one embodiment of the invention, the particles of the first alloy have a liquidus temperature and a solidus temperature which differ by not greater than 15° C., and the particles of the second alloy have a liquidus temperature and a solidus temperature which differ by not greater than 15° C. Preferably, the particles of the first alloy have a liquidus temperature and a solidus temperature which differ by not greater than 10° C., and the particles of the second alloy have a liquidus temperature and a solidus temperature which differ by not greater than 10° C. More preferably, the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 10° C. Most preferably, the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 5° C.

In another embodiment of the invention, the first alloy comprises Sn, Ag and at least one additional metal. Preferably, the at least one additional metal is selected from the group consisting of Cu, Zn, Bi, Ni, and In. More preferably, the second alloy comprises Sn and Ag.

In a further embodiment, the invention is directed to a mixed alloy lead-free solder paste, which comprises particles of a first alloy of Sn—Ag—Cu; particles of second alloy of Sn—Ag; and flux. The particles of the first and second alloy are mixed with the flux.

In another embodiment if the invention, the first alloy comprises from 2.3 wt. % to 4.3 wt. % Ag, and from 0.4 wt. % to 1.2 wt. % Cu, with the remainder being Sn. Preferably, the first alloy comprises from 2.5 wt. % to 4.1 wt. % Ag, and from 0.5 wt. % to 1.0 wt. % Cu, with the remainder being Sn.

In yet another embodiment, the second alloy comprises from 2.0 wt. % to 5.0 wt. % Ag, with the remainder being Sn. Preferably, the second alloy comprises from 2.3 to 4.2 wt. % Ag, with the remainder being Sn.

Further included in the invention is a method of making solder paste. In one embodiment, the method comprises providing particles of a first alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C.; and providing particles of a second alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C., wherein the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 15° C.; The particles of the first and second alloy are mixed with a flux to form a solder paste.

In one embodiment, the mix of particles in the solder paste is comprised of at least 50 wt. % of the first alloy and not greater than 50 wt. % of the second alloy. Preferably, the mix of particles in the solder paste is comprised of from 50 wt. % to 90 wt. % of the first alloy and from 10 wt. % to 50 wt. % of the second alloy.

The invention further includes a method of soldering an electrical component to a substrate. In one embodiment, the method comprises applying a mixed alloy lead-free solder paste to a substrate, the mixed alloy lead-free solder paste comprising particles of a first alloy of Sn—Ag—Cu; particles of second alloy of Sn—Ag; and flux, wherein the particles of the first and second alloy are mixed with the flux. An electrical component is placed on the mixed alloy lead-free solder paste, and the electrical component, substrate and solder paste are heated to melt and flow the first alloy and second alloy. The electrical component, substrate and melted first alloy and second alloy are then cooled to affix the electrical component to the substrate.

DETAILED DESCRIPTION OF EMBODIMENTS

This invention provides a solder paste, a method of making the solder paste, and a substrate and electrical component soldered together by the solder paste. The solder paste of the invention is a mixed alloy solder paste in which the alloys have similar melting characteristics. By using the soldering paste of the invention to solder an electrical component to a substrate, the soldered product will exhibit little, if any, tombstoning. That is, the electrical component will be relatively evenly adhered to the substrate, as opposed to having one end of the component be soldered to the substrate in a substantially inclined angle relative to one of its opposing ends.

The solder paste of the invention comprises particles of a first alloy composition and particles of a second alloy composition. The particles are mixed together with a flux to form the solder paste. The solder paste is then used to solder an electrical component to a substrate.

Desirably, the first alloy contained in the solder paste has a liquidus temperature and a solidus temperature which differ by not greater than about 20° C. Generally, the liquidus temperature is the lowest temperature at which the alloy is completely in the molten state. The solidus temperature is the highest temperature at which the alloy is completely in the solid or non-molten state. Preferably, the liquidus temperature and the solidus temperature differ by not greater than about 15° C.; more preferably, the liquidus temperature and the solidus temperature differ by not greater than about 10° C.

In one embodiment of the invention, the first alloy comprises Sn, Ag and at least one addition metal. The first alloy has a liquidus temperature of not greater than about 225° C., preferably not greater than about 222° C., and most preferably not greater than about 220° C.

The at least one additional metal in the first alloy is selected from the group consisting of Cu, Zn, Bi, Ni, and In. Preferably, the additional metal is Cu.

In one embodiment of the invention, the first alloy comprises Cu as a third metal, and a fourth metal, the fourth metal being one which does not substantially increase the difference between the solidus and liquidus temperatures. Preferably, the fourth metal is selected from the group consisting of Zn, Bi, Ni, and In. More preferably, the fourth metal is Bi. The fourth metal, when present, is present at a concentration of from about 0.1 wt. % to about 5 wt. %, based on total weight of the alloy.

In one embodiment of the invention, the first alloy comprises from about 2.3 wt. % to about 4.3 wt. % Ag, and from about 0.4 wt. % to about 1.2 wt. % Cu, with the remainder being Sn. Preferably, the first alloy comprises from about 2.5 wt. % to about 4.1 wt. % Ag, and from about 0.5 wt. % to about 1.0 wt. % Cu, with the remainder being Sn.

It is desirable that the second alloy have melting characteristics, which are similar to the first alloy. In this regard, it is desirable that the second alloy contained in the solder past have a liquidus temperature and a solidus temperature which differ by not greater than about 20° C. Preferably, the liquidus temperature and the solidus temperature differ by not greater than about 15° C.; more preferably, the liquidus temperature and the solidus temperature differ by not greater than about

In one embodiment of the invention, the second alloy comprises Sn and Ag. The first alloy has a liquidus temperature of not greater than about 225° C., preferably not greater than about 222° C., and most preferably not greater than about 220° C.

In one aspect of the invention, the second alloy comprises from about 2.0 wt. % to about 5.0 wt. % Ag, with the remainder being Sn. Preferably, the second alloy comprises from about 2.3 wt. % to about 4.2 wt. % Ag, with the remainder being Sn.

In another aspect of the invention, the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 15° C. Preferably, the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 10° C.; more preferably, the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 5° C.

The alloys used in this invention can be made by conventional means. In one embodiment, the alloys are made by mixing the appropriate portion of metals in a metal or ceramic crucible and heating the mixture until the metals become molten. The molten metal is then poured into molds, and ingots are made when the metal cools and solidifies.

The ingots can be made into alloy particles by any conventional means. Examples include gas atomization and centrifugal atomization. The particles desirably have an average particle size of from about 200 mesh to about 400 mesh. Finer particles can be used. The resulting particles are mixed with flux to form a solder paste.

The flux portion of the alloy paste of the invention can be any conventional flux material suitable for use in soldering electrical components to substrate materials. An example of such flux is a rosin flux. Preferably, the rosin flux is a non-water soluble, rosin-based flux. The rosin in the flux may be either a natural rosin or a modified rosin, including a polymerized rosin. The flux can also contain an activator and a solvent.

The solder paste of this invention is made by mixing together particles of the first and second alloy with the flux. Desirably the particle mixture is added to or mixed with the flux, and the particle mixture comprises at least 50 wt. % of the first alloy and not greater than 50 wt. % of the second alloy. Preferably, the particle mixture comprises from about 50 wt. % to about 90 wt. % of the first alloy and from about 10 wt. % to about 50 wt. % of the second alloy; more preferably from about 55 wt. % to about 85 wt. % of the first alloy and from about 15 wt. % to about 45 wt. % of the second alloy; and most preferably from about 60 wt. % to about 80 wt. % of the first alloy and from about 20 wt. % to about 40 wt. % of the second alloy.

The solder paste will generally comprise from about 42 wt. % to about 83 wt. % of the first alloy, from about 9 wt. % to about 42 wt. % of the second alloy, and from about 8 wt. % to about 16 wt. % of the flux. Preferably, the solder paste comprises from about 47 wt. % to about 77 wt. % of the first alloy, from about 14 wt. % to about 39 wt. % of the second alloy, and from about 9 wt. % to about 14 wt. % of the flux; more preferably the solder paste comprises from about 53 wt. % to about 72 wt. % of the first alloy, from about 18 wt. % to about 35 wt. % of the second alloy, and from about 10 wt. % to about 12 wt. % of the flux.

The solder paste of this invention is considered a lead-free solder paste. According to this invention lead-free means that the solder paste itself contains not greater than about 3,000 ppm lead, based on total weight of the paste. Preferably, the solder paste contains not greater than about 2,000 ppm lead; more preferably, the solder paste contains not greater than about 1,000 ppm lead, based on total weight of the paste.

The solder paste of the invention is applied to a substrate, such as a circuit board. Application can be by any conventional method such as by stencil or screen printing. Following application of the solder paste onto the substrate, an electrical component is placed onto the solder paste on the printed circuit board and this assembly is placed into an oven and heated to a temperature sufficient to cause the solder powder to melt and flow. When the temperature decreases, the molten solder solidifies forming solder joints with the electrical components. This type of method can be referred to as conventional reflow soldering.

Heating can be performed in two stages, if desired. In the two stage heating process, the temperature is kept relatively constant at a preheating temperature of from about 100° C. to about 170° C. Then the temperature is increased up to not higher than about 240° C., preferably not higher than about 230° C.

Following heating, the assembly is cooled to provide a substrate having an electrical component soldered thereto. Due to the composition of the alloy powders provided in this invention, problems such as tombstoning are substantially reduced or eliminated. The result is a substrate and electrical component combination in which the electrical component is affixed to the substrate by a homogeneous solder alloy.

The solder paste of this invention can be used to solder any number of electrical components to any number of suitable substrates. Electrical components include those used to surface mount assembly. These components have solderable terminations that are bonded to by the solder paste when the paste is metled to form a liquid, and the liquid is solified upon cooling. Examples of such electrical components include the Ball Grid Array (BGA), Thin Small Outline Package (TSOP), capacitors, resistors, air wound coils, and any others that are used in electronic assemblies.

The substrate used in this invention can be any type of substrate suitable for connecting with an electrical component. Such substrates include those that are solderable and able to be exposed to the reflow temperatures described in the invention. Generally, such substrates include ceramic and glass-epoxy materials, and can be of the flexible or rigid type. Particular examples of suitable substrates include Printed Wiring Boards and Ceramic substrates that are commonly found in electronic assemblies. Printed Wiring Boards are multi-layer composites of epoxy encasing glass fibers that contain an electrically conducting lines, pads and vias.

Using the solder paste and method of this invention will result in an electrical component that is securely affixed to a substrate by a lead-free solder material. The solder material will be essentially homogeneous and provide highly desirable solder characteristics. The hardened solder will comprise a mixture of phases comprising the elements in the solder alloys. The specific size and shape of the phases will depend on the cooling rate and specific final alloy composition after the two alloys are melted together and then solidified upon cooling.

Having now fully described this invention, it will be appreciated by those skilled in the art that the invention can be performed within a wide range of parameters within what is claimed, without departing from the spirit and scope of the invention. 

1. A mixed alloy solder paste comprising: particles of a first alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C.; particles of a second alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C.; and flux, wherein the particles of the first and second alloy are mixed with the flux and the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 15° C.
 2. The method of claim 1, wherein the particles of the first alloy have a liquidus temperature and a solidus temperature which differ by not greater than 15° C., and the particles of the second alloy have a liquidus temperature and a solidus temperature which differ by not greater than 15° C.
 3. The method of claim 1, wherein the particles of the first alloy have a liquidus temperature and a solidus temperature which differ by not greater than 10° C., and the particles of the second alloy have a liquidus temperature and a solidus temperature which differ by not greater than 10° C.
 4. The method of claim 1, wherein the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 10° C.
 5. The method of claim 1, wherein the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 5° C.
 6. The method of claim 1, wherein the first alloy comprises Sn, Ag and at least one additional metal.
 7. The method of claim 6, wherein the at least one additional metal is selected from the group consisting of Cu, Zn, Bi, Ni and In.
 8. The method of claim 1, wherein the second alloy comprises Sn and Ag.
 9. A mixed alloy lead-free solder paste comprising particles of a first alloy of Sn—Ag—Cu; particles of second alloy of Sn—Ag; and flux, wherein the particles of the first and second alloy are mixed with the flux.
 10. The mixed alloy lead-free solder paste of claim 9, wherein the first alloy comprises from 2.3 wt. % to 4.3 wt. % Ag, and from 0.4 wt. % to 1.2 wt. % Cu, with the remainder being Sn.
 11. The mixed alloy lead-free solder paste of claim 9, wherein the first alloy comprises from 2.5 wt. % to 4.1 wt. % Ag, and from 0.5 wt. % to 1.0 wt. % Cu, with the remainder being Sn.
 12. The mixed alloy lead-free solder paste of claim 9, wherein the second alloy comprises from 2.0 wt. % to 5.0 wt. % Ag, with the remainder being Sn.
 13. The mixed alloy lead-free solder paste of claim 9, wherein the second alloy comprises from 2.3 to 4.2 wt. % Ag, with the remainder being Sn.
 14. A method of making solder paste, comprising: providing particles of a first alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C.; providing particles of a second alloy having a liquidus temperature and a solidus temperature which differ by not greater than 20° C., wherein the liquidus temperature of the first alloy and the liquidus temperature of the second alloy differ by not greater than 15° C.; and mixing the particles of the first and second alloy with a flux to form a solder paste.
 15. The method of claim 14, wherein the mix of particles in the solder paste is comprised of at least 50 wt. % of the first alloy and not greater than 50 wt. % of the second alloy.
 16. The method of claim 14, wherein the mix of particles in the solder paste is comprised of from 50 wt. % to 90 wt. % of the first alloy and from 10 wt. % to 50 wt. % of the second alloy.
 17. A method of soldering an electrical component to a substrate, comprising: applying a mixed alloy lead-free solder paste to a substrate, the mixed alloy lead-free solder paste comprising particles of a first alloy of Sn—Ag—Cu; particles of second alloy of Sn—Ag; and flux, wherein the particles of the first and second alloy are mixed with the flux; placing an electrical component on the mixed alloy lead-free solder paste; heating the electrical component, substrate and solder paste to melt and flow the first alloy and second alloy; and cooling the electrical component, substrate and melted first alloy and second alloy to affix the electrical component to the substrate.
 18. The method of claim 17, wherein the first alloy comprises from 2.3 wt. % to 4.3 wt. % Ag, and from 0.4 wt. % to 1.2 wt. % Cu, with the remainder being Sn.
 19. The method of claim 17, wherein the first alloy comprises from 2.5 wt. % to 4.1 wt. % Ag, and from 0.5 wt. % to 1.0 wt. % Cu, with the remainder being Sn.
 20. The method of claim 17, wherein the second alloy comprises from 2.0 wt. % to 5.0 wt. % Ag, with the remainder being Sn.
 21. The method of claim 17, wherein the second alloy comprises from 2.3 to 4.2 wt. % Ag, with the remainder being Sn. 